Victory at the Stadio Artemio Franchi would guarantee the Rossoneri Champions League football, but fourth-placed Fiorentina are ready to capitalise on any slip-up when they visit relegated Pescara.
Reports in Italy suggest Allegri could call time on his three-year tenure in Milan to join Roma, but the Tuscan would not lend credence to such hearsay at today's press conference.
He told acmilan.com: "We want to end the season on a high note and we want to finish third to qualify for the Champions League.
"The players have trained well all week and everyone concerned with the club deserves to be in the Champions League. The fans have really got behind us and there is plenty of enthusiasm within the club.
"The players are relaxed and we can look forward to a good game. We have two points more than Fiorentina and it is just a pity that one of us will not qualify for the Champions League because they have had a good season.
"I am fully focused on the match against Siena so speaking about my future does not serve anyone. I will speak to the club after tomorrow's match and my phone calls with the president have always been cordial.
"I speak to (vice-president Adriano) Galliani every day about the objectives we want to reach."
Midfielder Sulley Muntari serves a suspension following his bizarre red card against Roma last weekend, while Nigel De Jong, Bojan and Kevin-Prince Boateng miss out through injury.
Allegri dropped several hints about his starting XI at Siena, saying: "(Riccardo) Montolivo trained normally and he could start so it will be between (Mathieu) Flamini, (Massimo) Ambrosini and (Antonio) Nocerino for the other two places.
"Robinho could start in attack and (Giampaolo) Pazzini could play through the middle with (Mario) Balotelli moving out wide.
"We want to play well and we cannot dwell on the fact that Siena are already relegated."
Siena are unable to call on the injured trio of defender Carlos Matheu, midfielder Simone Vergassola and striker Nicola Pozzi.
Bianconeri boss Giuseppe Iachini, meanwhile, claimed his club were always on the backfoot having been issued with a six-point penalty for involvement in match-fixing in previous seasons.
"We go down with our heads held high, with great dignity," he told La Repubblica. "With the points penalty we have always been forced to play catch-up."